Capacity Control Methods

1. Overview

Process conditions rarely remain constant. Reciprocating compressors must adapt to varying inlet pressures, temperatures, and flow requirements. Capacity control allows matching compressor output to actual demand while maintaining efficient operation.

Control Methods Comparison

2. Clearance Pockets

Clearance pockets are additional cylinder volumes that can be opened or closed to modify the effective clearance. Increasing clearance reduces volumetric efficiency and thus capacity.

How Clearance Affects Capacity

As clearance increases, more gas is trapped at the end of the discharge stroke. This gas must re-expand before suction can begin, reducing the volume available for new gas:

Pocket Types

Pocket Sizing Example

3. Valve Unloaders

Valve unloaders (or finger unloaders) hold suction valves open during the entire stroke, preventing any compression. The cylinder end produces zero capacity but also consumes minimal power.

Unloader Operation

• Pneumatic or hydraulic actuator pushes fingers through valve
• Fingers hold valve plates away from seat
• Gas flows freely in and out through suction valve
• No pressure rise, no compression work
• Small power consumption for friction and valve losses

Unloader Arrangements

Combining with Pockets

The most flexible capacity control combines valve unloaders with clearance pockets:

4. Speed Control

Varying compressor speed directly changes capacity since flow is proportional to RPM. This provides truly infinite capacity control but requires variable speed capability in the driver.

Speed Control Methods

Speed Range Limits

5. System Control Methods

Beyond cylinder-level control, system-level methods can regulate capacity by managing how gas flows through the compressor.

Bypass (Recycle) Control

A control valve returns discharge gas to suction, reducing net flow to the process while the compressor runs at full capacity:

• Simple to implement
• Infinite turndown possible
• Very poor efficiency (compressing same gas repeatedly)
• Requires cooler in bypass to remove heat of compression
• Best used for short-term upset conditions

Suction Throttling

Reducing suction pressure decreases gas density and thus mass flow:

• Simple valve at suction
• Increases compression ratio (higher power per unit mass)
• Limited range before efficiency drops severely
• May cause rod load or temperature issues

On/Off Control

For applications with storage (tanks, pipelines), cycling the compressor on and off can provide average capacity control:

• Simple, no special equipment
• Wear from frequent starts
• Pressure swings in system
• Best for low-hour applications

6. Load Step Planning

When designing a capacity control system, the goal is to provide enough load steps to match demand without excessive complexity or cost.

Planning Considerations

• Operating range: What capacity range must be covered?
• Control accuracy: How closely must output match demand?
• Rod load balance: Steps should maintain acceptable loading
• Power consumption: Minimize wasted energy at part load
• Thermal effects: Avoid excessive temperatures at any step

Multi-Cylinder Optimization

With multiple cylinders, careful selection of which ends to unload and which pockets to open provides many more load steps:

7. Power Consumption at Part Load

Different capacity control methods have very different power consumption characteristics at part load. Understanding this is crucial for energy optimization.

Power vs. Capacity

Hybrid Approaches

Many installations combine methods for optimal results:

• Valve unloaders for coarse steps
• Clearance pockets for fine adjustment
• Speed variation for continuous trim
• Bypass for surge protection only